Added opacity.

Perspex.Direct2D1/Renderer.cs

@@ -120,8 +120,16 @@ namespace Perspex.Direct2D1
         /// <param name="context">The drawing context.</param>
         private void Render(IVisual visual, DrawingContext context)
         {
-            if (visual.IsVisible)
+            if (visual.IsVisible && visual.Opacity > 0)
             {
+                if (visual.Opacity < 1)
+                {
+                    Layer layer = new Layer(this.renderTarget);
+                    LayerParameters p = new LayerParameters();
+                    p.Opacity = (float)visual.Opacity;
+                    this.renderTarget.PushLayer(ref p, layer);
+                }
+
                 visual.Render(context);
 
                 foreach (IVisual child in visual.VisualChildren)
@@ -133,6 +141,11 @@ namespace Perspex.Direct2D1
                         this.Render(child, context);
                     }
                 }
+
+                if (visual.Opacity < 1)
+                {
+                    this.renderTarget.PopLayer();
+                }
             }
         }
     }

Perspex/IVisual.cs

@@ -18,6 +18,8 @@ namespace Perspex
 
         bool IsVisible { get; }
 
+        double Opacity { get; }
+
         IEnumerable<IVisual> VisualChildren { get; }
 
         IVisual VisualParent { get; set; }

Perspex/Layout/Layoutable.cs

@@ -130,7 +130,10 @@ namespace Perspex.Layout
 
         public void Arrange(Rect rect)
         {
-            this.ArrangeCore(rect);
+            if (this.DesiredSize.HasValue)
+            {
+                this.ArrangeCore(rect);
+            }
         }
 
         public void InvalidateMeasure()

Perspex/Visual.cs

@@ -19,6 +19,9 @@ namespace Perspex
         public static readonly PerspexProperty<bool> IsVisibleProperty =
             PerspexProperty.Register<Visual, bool>("IsVisible", true);
 
+        public static readonly PerspexProperty<double> OpacityProperty =
+            PerspexProperty.Register<Visual, double>("Opacity", 1);
+
         private IVisual visualParent;
 
         private Rect bounds;
@@ -34,6 +37,12 @@ namespace Perspex
             set { this.SetValue(IsVisibleProperty, value); }
         }
 
+        public double Opacity
+        {
+            get { return this.GetValue(OpacityProperty); }
+            set { this.SetValue(OpacityProperty, value); }
+        }
+
         Rect IVisual.Bounds
         {
             get { return this.bounds; }